Study on the raising technique using one blast based on the combination of long-hole presplitting and vertical crater retreat multiple-deck shots

Abstract

This paper reports the further study carried out by the authors on the technique of long-hole raising in one blast. In the previous work reporting the results of preliminary study, method of vertical crater retreat (VCR) multiple-deck shots with different and smaller slice heights was used to excavate the raise in one blast. A 32-m raise in the cover of an abandoned cavity was successfully broken through. In this paper, an improved technique is suggested to overcome the overbreak and freezing in a long-hole raising blast. In the raising process, long-hole presplitting is first performed at the periphery of the blasting pattern, and then inner VCR multiple-deck shots with the same slice height are conducted according to a specific initiation sequence without in-slice delays. The previously calibrated numerical model is used to simulate the blasting process. The rock mass behaviours and the raising mechanisms are examined. It is shown that after the introduction of long-hole presplitting, the overbreak and freezing are greatly relieved, and the direction of advance and timing pattern are vitally responsible for the evolution of rock damage. This suggested technique was then applied to field raising blasts, and a 29-m raise and a 24-m raise under special cavity conditions were successfully developed. It demonstrates that, compared to the method of purely VCR multiple-deck shots, the design process can be greatly simplified and the present technique leads to better blasting performances.